Fans are used as means for driving air through ventilation systems. Efficiency and a low level of noise are requirements upon fans. The most common type of fans in the field of ventilation, are double suction radial fans. They have a high degree of efficiency but the level of noise requires subsequent silencers. A problem with double suction radial fans, is that the outlet velocity is high and uneven. This makes it impossible to place subsequent silencers immediately in connection to the fan. In recent time, chamber fans which is an old design, has come back to the market. They have the advantage that they provide a flow having a low and fairly even velocity. The fan is compact and a silencer may be placed closer to the fan. The lower maximum degree of efficiency for a chamber fan is compensated by the lower velocity and the accompanying lower pressure losses, so that for high flows, chamber fans obtains higher efficiency than double suction radial fans.
Fans are the cause of low frequency noise which is difficult to muffle. It is true that the level of noise in a ventilation system may be reduced by the use of silencers. However, this is a space consuming and expensive solution of the problem.
The primary sound source in a chamber fan is the fan wheel, which for example generates noise in the frequency range 50-300 Hz. The periphery of the fan wheel is preferably symmetrically shaped with blade edges extending substantially in parallel with the rotation shaft of the wheel. The blades are located between two lateral to said shaft arranged, mutually substantially parallel end plates. The generation of noise is predominantly caused by turbulence being created when the air is pressed radially out of the wheel by the rotation of the blades, and then is brought to flow on, e.g. axially, by the design of the chamber walls.